Starch-based adhesives have been used extensively in the production of corrugated paperboard and corrugated containers. However, commonly utilized starch adhesives are not normally water resistant. Conventional starch based corrugating adhesives include cooked or gelatinized starch and raw, i.e., unmodified starch particles. The cooked starch functions as a carrier and serves to suspend the unmodified starch granules. The pH of these adhesives is 10 or greater, preferably 12, in order to reduce the gelation temperature of the raw starch. Typically, borax is included to speed gelation. This gelation, along with the interaction of borax and carrier, causes these compositions to function as adhesives upon heating during the corrugation process. The high pH and subsequent solubilization of the unmodified starch after heating result in the poor water resistance of conventional starch adhesives.
The most common means of reducing this water sensitivity involves addition of a urea-aldehyde or ketone-aldehyde resin to insolubilize the adhesive. These resins are base-catalyzed and because of the high pH needed, the pot life of these formulations is short--12 hours or less. Care must also be taken to apply and to cure them properly during the corrugating process. A heavy application of adhesive and a hot stack curing time of 24 hours or greater are commonly required in using these adhesives to provide corrugated products.
Another alternative which has been employed to render the adhesive water-resistant is to lower of the pH of the adhesive to the acid side (pH 3.5-4.5) with a catalyst such as alum or ammonium sulfate while raising the solids from the normal 20-22% to 28-32%. An acid cured, insolubilizing resin is added, urea-formaldehyde or melamine-formaldehyde, for example. An adhesive which is considerably more water resistant than the alkaline system described initially is formed. The greatest disadvantage to this type of adhesive is the limited corrugating speed resulting from the acid pH and consequent high gelling temperature of the adhesive.
As a third alternative, essentially the same system is used with the addition of a polyvinylacetate latex and a further increase in solids to 43-47%. This system increases the corrugating speeds which may be used. However, the low pH of the adhesive results in corrosion of corrugating equipment. Moreover, these adhesives normally have a pot life of only 24 hours or less. The adhesives are often incompatible with regularly run water resistant conventional starch adhesives, requiring separate handling systems and time consuming rinsing of corrugating equipment. Futhermore, the cooked starch portion of the adhesive is prone to premature gelling or "setting back" requiring heating and agitation of the adhesive during storage.